Blowout preventers for oilfield tubulars have been used for decades by operators of hydrocarbon recovery wells. U.S. Pat. Nos. 1,875,673, and 1,949,672 disclose early types of pressure control equipment for a well capping and extinguishing a fire at the surface of an oil or gas well.
Powered shear ram equipment has been commercially available since at least the early 1960's for shearing off an oilfield tubular, and for sealing against the tubular when the opposing rams are closed. U.S. Pat. No. 3,561,526 discloses the use of overlapping knife blades to perform the shearing function. U.S. Pat. No. 3,736,982 discloses a blowout preventer wherein the knife blade shearing rams are spaced above and are operable separately from the sealing rams of the blowout preventer. As shown in the '526 patent, the knife blades may be concave to include tubular engaging surfaces which taper slightly inwardly from the sides, so that the knife blades center the tubular as the rams move in. The tubular is sheared by first substantially flattening the pipe, and the subsequent shearing of the tubular thereafter occurs, as disclosed in this patent. Variations of this equipment include a single shear ram, as disclosed in U.S. Pat. No. 3,590,920.
A great deal of effort has been expended to enhance the utility and effectiveness of powered shear ram equipment for blowout preventers (BOPs). U.S. Pat. No. 4,313,496 provides a reciprocating device which is powered to cause the cutting blades to shear large diameter tubulars, such as casing and drill collars. This patent also discloses arms to resist the forces tending to vertically separate the cutting blades during the shearing operation. U.S. Pat. No. 4,540,046 discloses improvements in the shearing blade and ram block subassembly to reduce the thickness of the ram block, so that the opposing ram assemblies can seal off high pressure fluids. Other significant improvements have been made to enhance the reliability and operation of rams for the blowout preventer (BOP). U.S. Pat. No. 5,025,708 discloses an automatic lock for a ram actuator to prevent inadvertent opening of ram blocks, thereby increasing safety.
In spite of the improvements referenced above, hydrocarbon recovery operators have continued to desire equipment and techniques which more effectively and more reliably shear oilfield tubulars. U.S. Pat. No. 4,923,008 discloses a hydraulic power system specifically designed for providing the desired high pressure driving force to the ram pistons of a blowout preventer. This system initially provides sufficient power to substantially close the rams, after which time the system releases a high pressure force to cause the final shearing of the oilfield tubular.
One of the significant problems relating to shearing an oilfield tubular extending into a wellbore concerns the size of the blowout preventer body, which defines the through passageway for receiving a tubular of a maximum size. In a typical application, a blowout preventer body having an 183/4 inch bore is only able to effectively and reliably shear an oilfield tubular having less than a 103/4 inch diameter, since the flattening out of the tubular during the shearing process would otherwise cause the edges of the flattened tubular to be forced into binding engagement with the side walls of the BOP passageway, thereby adversely affecting the safety and reliability of the shearing and BOP sealing operation.
The conventional shearing of an oilfield tubular by a shearing ram assembly by first substantially flattening the tubular also creates problems for subsequent operations. Since the top of the lower tubular still within the wellbore has been flattened, it is difficult to thereafter pump a plugging material into the lower tubular to "kill" the well. Also, the flattened top of the lower tubular is difficult to retrieve by a conventional fishing operation, particularly since the flattened end corners are spaced apart a distance substantially greater than the tubular diameter, and tend to catch on the sidewalls of the BOP.
The disadvantages of the prior art are overcome by the present invention, and an improved blowout preventer and techniques for shearing a oilfield tubular extending into a wellbore are hereinafter provided. The present invention discloses a reliable technique for shearing an oilfield tubular, which can generally be accomplished with less power being supplied to the shearing rams compared to prior art shearing techniques. Moreover, the apparatus of the present invention is able to effectively shear a comparatively larger diameter oilfield tubular, so that a blowout preventer body having an 183/4 inch bore may effectively shear a tubular having a diameter of up to approximately 163/4 inches.